Reliability of ultrahigh sensitivity optical fiber sensors embedded in graphite composites

Fiber cavity etalon (FCE) sensors have demonstrated ultrahigh static strain sensitivity (~1 ne) when they are either surface-mounted to, or embedded, in graphite reinforced resin composites. Although a significant amount of data has been acquired at very low strain, little is known about their performance and durability in typical installations. Graphite/epoxy composite test specimens were fabricated to address practical concerns and to evaluate the reliability of embedded FCE sensors. Two different specimen configurations using two different composite fabrication methods were selected for sample installations: thin flat laminates and cylindrical struts. After fabrication, the FCE sensors were interrogated to ensure that they were still intact, to record a baseline response, and to determine any changes in response that might have occurred during manufacturing. Next, to determine the survivable strain limits of the embedded sensor, the specimens were loaded in tension to a predetermined strain level, unloaded, and then interrogated. Once these limits were found, the specimens were subjected to cyclic loading and periodically interrogated until sensor failure. The results from these tests provide practical strain limits for the embedded FCE sensor and show that the response does not change as a result of tensile cyclic loading.